Revert "Add StringSink and update StringBuffer."

sdk/lib/core/string_sink.dart

-// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
-// for details. All rights reserved. Use of this source code is governed by a
-// BSD-style license that can be found in the LICENSE file.
-
-part of dart.core;
-
-abstract class StringSink {
-
-  /**
-   * Converts [obj] to a String by invoking [:toString:] and adds the result to
-   * this [StringSink].
-   */
-  void write(Object obj);
-
-  /**
-   * Converts [obj] to a String by invoking [:toString:] and adds the result to
-   * this [StringSink]. Then adds a new line.
-   */
-  void print(Object obj);
-
-  /**
-   * Writes the [charCode] to this [StringSink].
-   *
-   * This method is equivalent to [:write(new String.fromCharCode(charCode)):].
-   */
-  void writeCharCode(int charCode);
-}